This invention relates to the treatment of liquids, especially oxygen enrichment of water which is utilized for aquaculture purposes such as fish breeding.
In aqueous environments for this purpose it is important to have available clean water, rich in oxygen and of high quality. According to the prior art it has been conventional to bubble air through the water to keep the oxygen content within a range of from 80-100% of saturation. It has, however, recently been proved that it is advantageous to increase the oxygen content to a super saturation of from 120-250%. This increases the growth rate substantially and also makes it possible to obtain a higher degree of utilization of a given breeding volume. Instead of using air or an oxygen containing gas for enrichment of water, it is also conventional to use pure oxygen as this results in less contamination and greater effect.
It is also important that there be no super saturation of nitrogen in the water. Already, at 3-6% of super saturation, there is a risk that the fish will suffocate. This can be critical, especially if heated water is utilized for fish breeding, as the amount of gas saturation in water will decrease with increasing temperature of the water.
Correspondingly, it is of equal importance to provide a low content of nitrogen and eventually to reduce such content through treatment of the water, as it is to have a sufficiently high content of oxygen.
From U.S. Pat. No. 4,116,164 is known a method for enrichment of the oxygen content in water in a fish breeding reservoir where an oxygen-containing gas, containing at least 25 percent by volume of oxygen, is introduced into the water reservoir and where the gas is bubbled through and distributed into the water to maintain a total dissolved oxygen level of at least 120% of super saturation in relation to ambient pressure and temperature. This requires the use of excess of an gas. That amount of gas which is not dispersed and dissolved into the water will rise to the surface where it is collected by means of a gas hood or the like. After purification and addition of fresh oxygen, the gas is returned to the water reservoir.
Such a method has, however, a number of drawbacks. The rising gas bubbles will create a form of ejector action and this will cause contamination due to uprising of excrements from the bottom. The lower specific weight of the water in the area where the gas bubbles are rising results in a decreased volume for utilization for breeding, because fish and fish fry will avoid the area where the bubbles are rising. The amount of water available is often a critical factor, especially when there are restricted amounts of water for utilization.